Defects in vision include myopia, hypermetropia and astigmatism. These are all very common defects, and are readily correctable by spectacles or contact lenses, which may be “rigid” or “soft”. Rigid contact lenses have a water content of less than 5%, whilst soft contact lenses generally have a water content of 20% or more.
Defects in human vision may also arise from other causes including: corneal transplants, accident, post laser eye surgery problems and ocular diseases including keratoconus. The latter condition is characterised by a localised thinning of the cornea, which leads to outward bulging of the cornea due to the pressure exerted thereon by the fluid between the cornea and the lens of the eye. This bulging of the cornea causes it to depart from the ideal asphericity, and so causes defective vision.
Keratoconic visual defects cannot be satisfactorily corrected by spectacles. Contact lenses have been used to help patients suffering from reduced visual acuity due to abnormal corneal surfaces caused by diseases such as keratoconus, post-operative corneal trauma and accidental corneal trauma.
Generally, conventional practice has been to prescribed patients with keratoconic defects hard contact lenses, especially “rigid gas permeable” (RGP) lenses. Because of their rigidity, these lenses can bridge over the distortions of the cornea, allowing the tears to fill voids between the lens and cornea. The refractive index of the tear is considered, by conventional wisdom, to be similar to, but not exactly the same as, both the front of the cornea and the contact lens. The anterior surface of the contact lens replaces the front of the cornea and becomes the main refractive element of the patient's eye. It is therefore important that this surface remains optically well defined.
This conventional method therefore relies on a contact lens being rigid enough to support itself over the distortions of the cornea, resisting the capillary forces of the tear layer, and not allowing these distortions to be transferred to the front surface of the contact lens. Therefore RGP contact lenses are mainly used to ensure that the lens retains its form on the eye.
However, the requirement that the contact lens needs to be rigid, coupled with the abnormal distortion of the cornea, means that the fitting of such rigid lenses for subjects of this sort is extremely difficult. The contact lens fitter is faced with sometimes insurmountable conflicting requirements of: (1) achieving good optical performance, (2) reducing the traumatic effects of the contact lens on the cornea, and (3) providing a lens to the patient that is tolerably comfortable, and (4) ensuring that no damage is done to the cornea as a rigid material on thinning tissue can cause abrasion and scarring.
To achieve this fit, it is typical for the contact lens fitter to use many trial lenses to review the fit and repeat order a number of specialist lenses from contact lens manufacturers. This is clearly inefficient, expensive and time-consuming, and can cause a lot of discomfort for the subject.
Accordingly, the present inventors have explored alternatives which, contrary to received wisdom, utilise soft contact lenses. There are some prior publications which disclose the design of soft contact lenses to treat, inter alia, keratoconic defects. These include WO2009/053755.
Intuitively, those skilled in the art would consider that, in general terms, the thicker such a soft lens is, the better its optical performance, as being better able to withstand the forces placed on it by the irregular shape of the patient's cornea. Equally, those skilled in the art would intuitively consider that, the more rigid the lens, the better its optical performance in situ, for the same reasons.